This invention relates in general to squaraines, and more specifically, to a process for preparing finely divided squaraine particles.
Squaraine compositions are useful for incorporation into photoresponsive devices to extend the response capability of such devices to visible light as well as infrared illumination. These photoresponsive devices can therefore be utilized, for example, in conventional electrophotographic copiers as well as in laser printers. These photoresponsive devices may comprise single or multilayered members containing photoconductive materials comprising squaraine compositions in a photogenerating layer, between a photogenerating layer and a hole transport layer, or between a photogenerating layer and a supporting substrate.
Photoconductive imaging members containing certain squaraine compositions, including amine derivatives of squaric acid, are known. Also known are layered photoresponsive devices containing photogenerating layers and transport layers, as described, for example in U.S. Pat. No. 4,123,27, U.S. Pat. No. 4,353,971, U.S. Pat. No. 3,838,095, and U.S. Pat. No. 3,824,099. Examples of photogenerating layer compositions disclosed in U.S. Pat. No. 4,123,270 include 2,4-bis-(2-methyl-4-dimethylamino-phenyl)-1,3-cyclobutadiene-diylium-1,3-d iolate, 2,4-bis-(2-hydroxy-4-dimethylamino-phenyl)-1,3-cyclobutadiene-diylium-1,3- diolate, and 2,4-bis-(p-dimethylamino-phenyl)-1,3-cyclobutadiene-diylium-1,3-diolate. Still other photoconductive imaging members containing certain squaraine compositions are disclosed, for example, in U.S. Pat. No. 4,490,452.
The formation and development of electrostatic latent images on the imaging surface of photoconductive members by electrostatic means is well known. Generally, the method involves the formation of an electrostatic latent image on the surface of an electrophotographic plate, referred to in the art as a photoreceptor. This photoreceptor usually comprises a conductive substrate and one or more layers of photoconductive insulating material. A thin barrier layer may be interposed between the substrate and the photoconductive layer in order to prevent undesirable charge injection.
Many different photoconductive members are known including, for example, a homogeneous layer of a single material such as vitreous selenium, or a composite layered device containing a dispersion of a photoconductive composition. An example of one type of composite photoconductive member is described, for example, in U.S. Pat. No. 3,121,006. The composite photoconductive member of this patent comprises finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. The photoconductive inorganic compound usually comprises zinc oxide particles uniformly dispersed in an electrically insulating organic resin binder coated on a paper backing. The binder materials disclosed in this patent comprise a material which is incapable of transporting for any significant distance injected charge carriers generated by the photoconductive particles. The photoconductive particles must therefore be in substantially contiguous particle to particle contact throughout the layer to permit the charge dissipation required for a cyclic operation. The uniform dispersion of photoconductive particles requires a relatively high volume concentration of photoconductive material, usually about 50 percent by volume, in order to obtain sufficient photoconductor particle to particle contact for rapid discharge. This high photoconductive particle loading can adversely affect the physical continuity of the resinous binder thereby significantly degrading the mechanical properties thereof. Specific binder materials disclosed in this patent include, for example, polycarbonate resins, polyester resins, polyamide resins, and the like.
Also known are photoreceptor materials comprising inorganic or organic materials wherein the charge carrier generating, and charge carrier transport functions are accomplished by discrete contiguous layers. Additionally, layered photoreceptor materials are disclosed in the prior art which include an overcoating layer of an electrically insulating polymeric material. However, the art of xerography continues to advance and more stringent demands need to be met by the electrostatographic imaging apparatus in order to improve performance, and to obtain higher quality images. Also desired are layered photoresponsive devices which are responsive to visible light and/or infrared illumination for certain laser printing applications.
Other layered photoresponsive devices including those comprising separate generating and transport layers are described, for example, in U.S. Pat. No. 4,265,990. Overcoated photoresponsive materials containing a hole injecting layer, overcoated with a hole transport layer, followed by an overcoating of a photogenerating layer, and an outer coating of an insulating organic resin are described, for example, in U.S. Pat. No. 4,251,612. Photogenerating layers disclosed in these patents include, for example, trigonal selenium and phthalocyanines and transport layers including certain diamines. The disclosure of U.S. Pat. Nos. 4,265,990 and 4,251,612 are incorporated herein by reference in their entirety.
In Belgium Pat. No. 763,540, an electrophotograhic member is disclosed having at least two electrically operative layers, the first layer comprising a photoconductive layer which is capable of photogenerating charge carriers and injecting the carriers into a continuous active layer containing an organic transporting material which is substantially non-absorbing in the spectral region of intended use, but which is active in that it allows the injection of photogenerated holes from the photoconductive layer and allows these holes to be transported through the active layer. Additionally, in U.S. Pat. No. 3,041,116, a photoconductive material is described containing a transparent plastic material overcoated on a layer of vitreous selenium contained on a substrate.
Squaraine pigments are an important group of infrared sensitive photoconductive pigments active in the GaAlAs solid state laser region (825 nm). Unsymmetrical squaraines usually have better electrical properties, but are difficult to prepare in small particle sizes. Large particles tend to cause defects in xerographic prints. Moreover, large particles do not adequately capture light efficiently and therefore require greater amounts in the binder layer. Difficulty in attaining small particle size unsymmetrical squaraine pigments also impacts pigment dispersion stability of the binder generator layer material. Pigments of unsymmetrical squaraines as synthesized, generally consist of crystals greater than about 20 .mu.m. Efforts by ball-milling to reduce particle size to usable levels have failed. Attrited particle size reduction has been somewhat successful in reducing particle size but the resulting pigments exhibit an increased dark decay. Efforts to reprecipitate squaraines such as Bis(4-dimethylaminophenyl)-1,3-cyclobutadienediylium-1,3-diolate, Bis(2-fluoro-4-dimethylaminophenyl)-1,3-cyclobutadienediylium-1,3-diolate and physical mixtures of Bis(4-dimethylaminophenyl)-1,3-cyclobutadienediylium-1,3-diolate and Bis(2-methyl-4-dimethylaminophenyl)-1,3-cyclobutadienediylium-1,3-diolate (a methyl squaraine) have yielded pigments with unacceptable dark decay values. Also, "spoiled" batches of squaraines are a major concern in any scale-up operation for producing squaraines.
While photoresponsive devices containing the above-described known squaraine materials are suitable for their intended purposes, there continues to be a need for the development of an improved process for preparing finely divided squaraine materials which exhibit low dark decay and high sensitivity.